The present invention relates to a feedback dimming control circuit, and more particularly, to a feedback dimming control circuit employing a frequency control technique for obtaining electronic ballast.
As shown in FIG. 1, a dimming control circuit employing a conventional open loop dimming technique comprises an oscillator 10 generating an oscillating signal having frequency f; a tank 12 which resonates at oscillation frequency f to generate a power supply voltage (or current) directly depending on the level of an input voltage V.sub.in ; and, a lamp 14 driven by tank 12. A parallel RC combination of a dimming resistor R.sub.f and a capacitor C.sub.f is connected to oscillator 10. Oscillator 10 is controlled by adjusting the value of dimming resistor R.sub.d to determine oscillation frequency f. With this circuit arrangement, the power supply provided by tank 12 to operate lamp 14 is also dependent on the exact adjustment of oscillating frequency f, i.e., the value of dimming resistor R.sub.d.
Unfortunately, the conventional dimming technique can not maintain a constant power supply output, and thus constant light luminance, in the face of fluctuations in input voltage V.sub.in or changes in the lamp load (e.g., a change in the number of lamps). Furthermore, proper dimming control can be lost in the foregoing circuit, such that desired luminance for a given environment cannot be maintained.